Magnesium oxide is an important ceramic material, used as a substrate, coating material, or for its refractory properties. It may also form under natural conditions, and iron-enriched MgO is known to be an important mineral phase belonging to the Earth's lower mantle, where it deforms at high pressure and high temperature. The mechanisms for its deformation are expected to depend strongly on the conditions of pressure, temperature, and strain rate. Among the defects that contribute to deformation, grain boundaries are known to be very sensitive to pressure.